Mould assembly provided with centering means

ABSTRACT

Mould assembly ( 1 ) consisting of two interacting mould sections ( 2, 3 ). One mould section is provided with a seat ( 4 ) and the other mould section with a body ( 5, 6 ) that is accurately accommodated by said seat ( 4 ), the body/seat interaction having a centring effect. The seat ( 4 ) is delimited by a series of ridge-shaped elements ( 8 ) that arm arranged according to an annular shape. The rigidity thereof in the radial direction is greater than that in the axial direction.

[0001] The present invention relates to a mould assembly comprising twomould sections which can be moved relative to one another and whichdelimit a mould cavity between them for injecting a plastic therein,which mould sections are provided with mutually interting centing means,said centring means of one mould section comprising an annular seat andsaid centring means of the other mould section comprising an annularbody, said body being embodied so as to fit tightly around the seat.

[0002] In practice it is always necessary to centre the mould sectionsof, for example, injection moulding machins with respect to one anotherin order accurately to delimit the mould cavity. It has been observedthat movement of mould sections relative to one another in a directionother than the closing direction is undiserable, certainly wheninjecting plastic. In a wide variety of process plastic is introducedbefore the mould sections are completely closed, whilst, moreover,processes exist in which the mould cavity is never completly closed. Afew such are known as praegan/coining and venting. In the case ofcoining a quanity of plastic is introduced during closure, but this doesnot completely fill the mould cavity. Complete filing takes place duringclosure as a result of the reduction in size of the mould cavity. Inthis case of venting the mould opens after having been closed.

[0003] In all cases movement in the radial plane with respect to thedirection of closure is undiserable. This applies in particular wheninjecting products with high accuracy. Optical information carries, suchas CDs, DVDs and the like may be mentioned as an example. As theinformation density inceases, ever more stringent requirements are beingimposed on the peaks and troughs. Moreover, it has been found that withcertain plastics the clarity thereof is reduced by mutual movement ofthe mould sections in the radial plane.

[0004] In the state of the art as described above, centring takes placewith the aid of a cone. That is to say the body is of conicalconstruction, whilst the seat is provided with a corresponding conicalopening. However, in theory such a centring functions only in oneposition of the mutual movement of the mould sections, that is to saythe position in which these parts touch one another. If contamination ispresent between the cone and the seat, the cone is pushed in onedirection by said contamination and accurate centring can no longer beguaranteed.

[0005] In order to eliminate this disadvantage of centring only in oneposition, so-called block guides have been proposed in prior art. Withthese guides the mould sections are centred relative to one another overa longer path of the closing movement. However, the mechanical forcesneeded to achieve movement in such a guide are appreciable. After all,there may not be any play between the various parts becausse otherwiseoptimum guiding cannot be guaranteed. Moreover, there is the problemthat on heating one of the mould sections either the play increases orjamming takes place between the parts of the block guide movingreleative to one another, as a consequence of which the wear increasesappreciably.

[0006] The aim of the present invention is to obviate the diadvantagesdescribed above and to provided a guide for mould sections which iseffective over an appreciable portion of the closing stroke of the mouldsections with respect to one another and provides accurate guiding, withwhich no problems arise in the cas of temperature differences betweenthe mould sections and which does not disproportionately increase theforces for opening and closing the mould sections.

[0007] This aim is realised with a mould assembly as described above inthat at least said seat or said body comprises at least five groovesextending to the free end thereof in order to delimit at least fiveadjacent elements for making clamping contact, around the peripherythereof, with said body or said seat, wherein, in the non-engagedposition, the effective internal diameter of said seat is 1-100 μmsmaller than the effective external diameter of said body.

[0008] According to the invention at least the body or the seat isconstructed as a closed part in the form of a comb, wherein the comb ismade up of a series of adjacent ridge-shaped elements that are separatedfrom one another by grooves. With this arrangement the dimensions of theelements are chosen such that the strength in the radial direction isgreater than the strength in the tangential direction. On the otherhand, the ridge-shaped elements are plastically deformable to someextent under the opening and closing forces that arise. The variousfeatures are achieved in that the radial length is appreciably greaterthan the tangential length of each ridge-shaped element. Surprisingly,it has been found out that by this means a clamping force can be exertedon the other mould section that, on the one hand, is not so great thatmovement of the mould sections into and out of one another issubstanially impeded but, on the other hand, provides centring.Moreover, it has been found that if contamination or the like is presentbetween one of the ridge-shaped elements and the opposing mould section,only the ridge-shaped element concerned will deform and, as a result,centring still takes place in the desired manner.

[0009] Optimum “processing” is obtained if the difference in diameter isbetween 1 and 100 μm and more particularly between 1 and 50 μm.

[0010] As a result of the large number of elements, an error such as theabove contamination will be averaged out.

[0011] The part in the form of a comb can have any closed peripheralshape, such as polygonal. Preferably it is made annular and moreparticularly elliptical and according to a further preferred embodimentcircular.

[0012] In principle it is possible that according to the invention theparts moving relative to one another are in contact over the entirelength thereof. That is to say the ridge-like elements press, over theentire “height” thereof, against the other mould section.

[0013] According to an advantageous embodiment of the invention eitherthe ridge-shaped elements or the opposing part of the other mouldsection is provided with a thickening so that engagement takes placeonly along an annular line. That is to say, if the ridge-shaped elementsare provided with an thickening, which preferably is made close to thefree end thereof, when the other mould section is moved relative theretoa constant lever force is produced by the ridge-shaped elements.

[0014] If, on the other hand, the opposing part of the other mouldsection is provided with a thickening, this thickening will then slideover the ridge-shaped elements will shorten during said movement. Thatis to say, in case of a closing movement the lever length becomesincreasingly shorter and the centring force consequently increasinglygreater. A relatively low centring force can suffice at entering, whichforce increases as the mould sections are closed further with respect toone another.

[0015] The ridge-shaped elements can be arranged as a ring and act as aseat for a body to be accomodated therein. However, it is possible for abody to be arranged around these, that is to say for the ridge-shapedelements to be placed in a seat. Depending on the embodiment, movementof the ridge-shaped elements take place outwards or inwards.

[0016] According to the invention at least five ridge-shaped elementsmust be fitted. In practice, larger numbers of up to a few hundred willbe used.

[0017] The invention will be explained in more detail below withreference to illustrative embodiments shown in the drawings. In thedrawing:

[0018]FIG. 1 shows, diagrammatically, a mould section provided withcentring according to the invention;

[0019]FIG. 2 shows, diagrammatically, the interaction of two mouldsections according to the invention;

[0020]FIG. 3 shows a detail of the construction shown in FIG. 2; and

[0021]FIG. 4 shows a modified embodiment of te construction according toFIG. 3.

[0022] The centring of two mould sections 2 and 3 which form part of amould assembly 1 is shown in FIGS. 1-3. As can be seen from FIG. 1, thesecond mould second 3 is provided with a centring seat 4 delimited by alarge number, for example a few tens, of ridge-shaped elements 8 betweenwhich grooves 9 have been made.

[0023] The radial length of these ridge-shaped elements is indicated bya, whilst the tangential length is indicated by b. The radial directionis indicated by R and the tangential direction by T. In this example theridge-shaped elements are more rigid in the radial direction R than inthe tangential direction T. That is to say, these elements are easilyable to deflect laterally but are able to deflect in the radialdirection only with difficulty. The grooves can, for example, beproduced by spark erosion machining but also by any other method knownin the state of the art. The material from which these ridge-shapedelements are made will in general be the same as the material of themould section concerned. It will also be subjected to the same heattreatment. The dimensions of the ridge-shaped elements are so chosenthat movements of a few hundredths of a millimetre at the free endthereof are possible without plastic deformation.

[0024] The way in which a solid centring body 5 provided with a solidcentring sleeve 6 arranged around it is introduced into the centringseat 4 is shown in FIG. 2. This centring sleeve 6 can be provided with asurface coating 7. A mould cavity 11 is delimited between the mouldsections, When the centing body or the centring sleeve 6 is movedinwards into the ridge-shaped elements 8 elastic deformation of theridge-shaped elements 8 will take place as a result of the difference insize, the centring sleeve 6 being approximately 1-100 and moreparticularly approximately 1-5 μm larger in diameter than the centringseat 4. It has been found that such elastic deformation requiresrelatively little effort, as a result of which the force needed formoving the first and second mould sections 2 and 3 towards one anotheris relatively low. On the other hand this construction produces aparticularly large centring effect, as a result of which particularlyaccurate centring of the mould sections with respect to one another ispossible. The rigidity of the construction thus obtained is many timesgreater than the rigidity of the injecton moulding machine, so that therequirements imposed by the manufactures are more than met. It will beunderstood that centring takes place continuously over the entire pathfor moving the centring sleeve 6 and the centring seat 4 towards oneanother as the centring sleeve 6 and the ridge-shaped elements 8 engage.This is in contrast to the prior art where centring is achieved by meansof cones. As a result it is possible to carry out operations such ascoining and venting without there being the risk that the mould sectionsmove relative to one another in a plane perpendicular to the directionof closure.

[0025] A particular variant of the invention is shown in FIG. 3. Theridge-shaped elements 8 are provided with a peripheral thickening 10. Asa result the centring sleeve 6 will engage exclusively on thisboss-shaped thickening 10. A centring effect takes place continuously,always with the same force, because the lever constituted by theridge-shaped element, that is to say the distance from the thickening 10relative to the base of the second mould section, remains constant.

[0026] This is different in the case of the embodiment that is shown inFIG. 4. In this figure the centring sleeve is indicated by 16, thecentring ridge shown by 18 and the coating applied thereto by 17. Inthis case the thickening is indicated by 20 and is positioned on thecentring body. Engagement of the thickening 20 takes place at various“heights” of the ridge-shaped element shown, that is to say the leveraction thereof changes on closing and the centring force increases onclosing.

[0027] The structure described above is able in a simple manner toabsorb alignment errors and to provide compensation for temperaturedifferences. It is possible to absorb deviations of approximately 0-1 mmdepending on the number of blocks and the diameter of the seat. Thestrength of each of the ridge-shaped elements in the radial directionis, as indicated above, much grater than that in the tangentialdirection. A factor of at least five is mentioned by way of example. Ingeneral the critical part of the closing movement is approximately 1 mmor less. The construction described above can be used for the injectionmoulding of any article that can be envisaged in the state of the art,but is used in particular for the injection moulding of opticalinformation carriers and more particularly, DVDs. The mould isconstructed accordingly.

[0028] Although we invention has been described above with reference topreferred embodiments, it will be understood that numerous modificationscan be made thereto without going beyond the scope of the presentapplication. Such modifications are immediately obvious to those skilledin the art on reading the above description and fall within the scope ofthe appended claims.

1. Mould assembly (1) comprising two mould sections (2, 3) which can bemoved relative to one another and which delimit a mould cavity (11)between them for injecting a plastic therein, which mould sections areprovided with mutually interacting centring means, said centring meansof one mould section comprising a seat (4) and said centring means ofthe other mould section comprising a body (5, 6), said body beingembodied so as to fit tightly around the seat, characterised in that atleast said seat or said body comprises at least five grooves (9)extending to the free end thereof in order to delimit at least fiveadjacent elements (8) for making clamping contact around the peripherythereof, with said body or said seat, wherein, in the non-engagedposition, the effective internal diameter of said seat is 1-100 μmsmaller than the effective external diameter of said body.
 2. Mouldassembly according to claim 1, wherein said elements are provided closeto the free end with a peripheral thickening (10) engaging on the body.3. Mould assembly according to one of the preceding claims, wherein thefree end of said body or said seat is provided with a peripheralthickening (20) engaging on the elements.
 4. Mould assembly according toone of the preceding claim, wherein at least one of said contactsurfaces between said elements and said body or said seat has awear-resistant coating (7, 17) that reduces friction.
 5. Mould assemblyaccording to one of the preceding claims, wherein said seat/said body isof circular cross-section.
 6. Mould assembly according to one of thepreceding claims, wherein said elements are embodied such that themechanical strength thereof is greater in the radial direction (R) thanin the tangential direction (T).
 7. Mould assembly according to one ofthe preceding claims, wherein said body is essentially cylindrical. 8.Mould assembly according to one of the preceding claims, wherein saidseat is essentially cylindrical.
 9. Mould assembly according to one ofthe preceding claims, wherein said mould cavity is designed for theproduction of an optical information carrier.
 10. Mould section having aseat for receiving the body of a mould section interacting therewith,which mould section is constructed in accordance with one of thepreceding claims.